In some water treatment systems, it is necessary to pump water against a head, into a holding tank or upgrade, for examples. Flap gates are used in such systems as they are held closed by the head and can be opened by an adequate pumping pressure on water to be passed therethrough.
Such gates are large and their weight is an important factor in the cost of such an installation as the heavier the gate, the greater is its cost and also the higher are pumping costs. As far as I am aware, such flap gates as are used for the above or like purposes have flat, internally braced walls resulting in constructions which, while capable of withstanding both head and pumping pressures, are disproportionate in both weight and cost.